Fisher-Tropsch (F-T) synthesis, one of the most important gas to liquid (GTL) techniques, was first introduced by German chemists, Fischer and Tropsch, who developed the process of producing synthesized fuel from synthesis gas by coal gasification. GTL process comprises three steps such as reforming natural gas, conducting F-T synthesis reaction from synthesis gas and hydrotreatment of F-T products. Among these, F-T reaction is conducted by using iron and cobalt as catalyst at 200-350° C. under 10-30 atm, and may be described with four main reactions as follows.
(a) Chain Growth in F-T SynthesisCO+2H2→—CH2—+H2O ΔH(227° C.)=−165 kJ/mol
(b) MethanationCO+3H2→CH4+H2O ΔH(227° C.)=−215 kJ/mol
(c) Water Gas Shift ReactionCO+H2O→CO2+H2 ΔH(227° C.)=−40 kJ/mol
(d) Boudouard Reaction2CO→C+CO2 ΔH(227° C.)=−134 kJ/mol
For the F-T reaction, iron-based or cobalt-based catalysts are commonly used. Although the iron-based catalyst was used in the past, nowadays the cobalt-based catalyst is mainly used for increasing the production of liquid fuel and wax and improving the conversion. The iron-based catalyst is the most low-priced one among various F-T catalysts and causes relatively less production of methane at high temperature and relatively high selectivity of olefins among hydrocarbons. Products may be used as fuel, and also be used as raw material in chemical industry such as light olefins and alpha olefins. Besides hydrocarbons, a large amount of side-products are produced such as alcohols, aldehydes and ketones.
Further, iron-based low-temperature F-T catalyst is commercially available from Sasol, which is used mainly for producing Sasol wax, comprises Cu and K components as promoters, and is prepared by a precipitation method using SiO2 as a binder. High-temperature F-T catalyst from Sasol is prepared by melting magnetite, K, alumina, MgO, etc.
Although the cobalt-based catalyst is above 200 times higher-priced one than Fe catalyst, it has advantages in relatively high activity and stability, and also causes high yield of liquid paraffin based hydrocarbon, while producing relatively less amount of CO2.
Further, this can be used as a low-temperature catalyst because a large amount of CH4 is produced at high temperature, and should be well dispersed onto a stable support with high surface area such as alumina, silica and titania due to the high-priced cobalt. This is usually used in such a form that a small amount of noble metal promoters such as Pt, Ru and Re are additionally added.
The production of major products, straight-chain hydrocarbons is explained mainly by Schulz-Flory' polymerization kinetic scheme. In F-T process, products with higher boiling point than that of diesel fuel are first produced in the amount of more than 60%. Thus, diesel fuel is produced by processes following the hydrocracking process, and waxes are transformed into high-quality lubricant oil by dewaxing process.
A reforming process for treating reduced crude that applies to general oil refining plant has been improved so as to guarantee process reliability due to the improvement of catalyst and process technique. However, oil (or wax) from F-T synthesis is far different from raw material treated in a reforming process of oil refining plant in terms of shape, state and properties, and this difference requires an appropriate hydrocarbon reforming process to be selected. Examples of a process of treating primary products of F-T reaction include hydrocracking, dewaxing, isomerization and allylation. Major products of F-T reaction include naphtha/gasoline, middle distillate (high centane number), S- and aromatic-free liquid hydrocarbons, α-olefins, oxygenates, waxes and so on.
As a general method to disperse high-priced active ingredient, cobalt and other activity-promoting materials are added onto a support with high surface area such as alumina, silica and titania, thereby providing catalyst. Specifically, to improve the dispersion of cobalt, i.e., an active ingredient, a commercial catalyst is prepared by using the one-component or two-component (mixed) support. However, it has been reported that the activity of F-T reaction only changes slightly depending on the kind of the support when the particle size of cobalt is similar [Applied Catalysis A 161 (1997) 59]. On the contrary, the activity of F-T reaction has been reported to depend largely on the dispersion and particle size of cobalt component [Journal of American Chemical Society, 128 (2006) 3956]. However, there have been attempts to improve FTS activity and stability by pre-treating the surface of support, thereby changing the properties of supports.
For example, when cobalt-supported alumina is used, the surface property of gamma-alumina can change into boehmite, etc., by the water generation during the reaction. This increases the rate of cobalt component oxidation, and the activity and thermal stability of the catalyst can be decreased. As a way to overcome these problems, there has been reported a process of pre-treating alumina surface with silicon precursor, thereby increasing catalyst stability [PCT publication; WO 2007/009680 A1]. Furthermore, there is disclosed that the treatment of the surface with various metals such as magnesium, zirconium, barium, boron and lanthanium to increase hydrothermal stability of support [U.S. Pat. No. 7,071,239 B2]. Another way to improve the activity of F-T catalyst is a method the preparation of a porous bimodal pore-structured silica-alumina catalyst, thereby increasing the mass transfer rate of components with high boiling point produced during the F-T reaction [publication of U.S. patent application Ser. No. 20050107479 A1; Applied Catalysis A 292 (2005) 252].
However, these techniques contains complicated synthesis processes of materials such as a process of forming a porous bimodal pore-structured support by using polymeric substrate or by preparing alumina-silica supports with difference pore size and simply mixing them and a process of supporting active ingredients such as cobalt by using the porous bimodal pore-structured support.
Although silica support shows less strong cobalt-support interaction than alumina support, it causes the loss of cobalt species due to the formation of cobalt silicates and resulting in lowering F-T activity. A pre-treatment of silica surface with metal such as zirconium has been reported as an effective method to overcome this problem [European patent No. 0167215 A2; Journal of Catalysis 185 (1999) 120].
Although F-T catalysts prepared by the aforementioned processes have various specific surface areas, F-T reactivity is known as closely related to change in cobalt particle size, pore size distribution of support and reducibility of cobalt component. Processes have been reported for preparing F-T catalyst by using well-known methods on the support prepared by complicated synthesis processes.